Embodiments described herein relate generally to hydraulic fracturing of rock and, more particularly, to methods for increased hydrocarbon recovery through mineralization sealing of hydraulically fractured rock within the subsurface, followed by refracturing.
A hydrocarbon-bearing subsurface geological formation generally consists of various regions or zones having differing permeability and porosity, defining the characteristics of an oil and/or gas reservoir. Hydrocarbon drive fluid moving through the reservoir preferentially moves to and through regions of higher permeability, such as fractures, bypassing regions of lower permeability, and the oil contained therein. Generally, during oil production, only a portion of the oil contained in a subsurface geological formation is recovered by so-called primary recovery methods of natural reservoir drives and other forces present in the oil-bearing reservoir. Fractures and other high-permeability streaks within the formations may be selectively depleted of oil during production, causing premature entry and production of adjacent gas or water, inhibiting further oil production.
Hydraulic fracturing is commonly used with pressurized fluids, creating fractures in rock so that trapped gas or oil can be produced more easily. A solid proppant, such as sand, is usually left behind in the fractures to ensure that the fractures remain open to provide the necessary porosity and permeability in the formations. Horizontal drilling and more effective isolation of targeted formations along the well bore have allowed better access to thin, highly productive formations.
Even with effective horizontal drilling and hydraulic fracturing, recovery of oil and gas in shale and other formations is typically low. In April 2013, the U.S. Geological Survey estimated that up to 7.4 billion barrels of oil in the Bakken Shale from the Williston Basin in North Dakota and Montana could be recovered using current technology. This is about 4.4% of the 167 billion barrels of oil in place, estimated by the North Dakota Department of Mineral Resources. For shale throughout the world, the U.S. Energy Information Administration estimates overall recovery factors of 15% to 30% for shale gas and 3% to 7% for shale oil.
Poor production recovery after hydraulic fracturing is often because most of the hydrocarbon-bearing portions of reservoir rocks are unaffected by the fracturing process, as hydraulic fracturing affects a small percentage of the rock in a radius proximal to the well bore. Hence, the quantity of oil produced out of a well from a typical shale formation, such as the Bakken Shale, is limited. Moreover, the price of oil and/or other economic factors often reduce the life of a well. In some instances, secondary recovery technologies may be able to rejuvenate and increase the profits from such wells. For example, to gain access to untouched hydrocarbon-bearing zones in a producing well in a low permeability in a fractured rock, the reservoir rock can be fractured further (i.e., refractured); however, the original fractures need to be sealed so that new fractures can be formed to access the hydrocarbon contained therein.
For example, some known techniques for recovering hydrocarbons by hydraulic fracturing include injecting or delivering proppant materials and/or fracturing fluids, such as an acid solution, to fracture a subsurface geological formation. To access untouched hydrocarbon-bearing zones, a sealing material or solution of non-living chemicals, cements, slurries, gels, and/or polymers can be injected into the geological formation and, as such, amorphous solids fill the formation fracture void spaces, which can then allow for refracturing. For example, some known methods of sealing fractures with non-living chemicals, cements, slurries, gels, and/or polymers are described in U.S. Pat. No. 5,181,568. U.S. Pat. No. 5,273,115, and U.S. Pat. No. 8,141,638, and International Patent Publication No. WO 2013192399A2. These methods are limited, however, because such sealing materials often solidify within a relatively shallow distance from the well bore. Moreover, in some instances, known methods can result in an incomplete sealing of the fractures and the introduction of foreign material into the formation. For example, sealing of micro fractures or the like can be difficult or impossible using such known methods.
Thus, a need exists for improved methods for sealing hydraulically fractured rock and refracturing.